The present disclosure generally relates to a polycarbonate composition. The polycarbonate composition contains a covert additive which becomes visible upon application of ultraviolet radiation (“UV”). Also disclosed are methods for preparing and using the same.
Fluorescent dyes are commonly used as colorants for the coloration of polymers. They are also used to enhance or change color for design or safety purposes. Fluorescent dyes are soluble under processing conditions and are preferably completely dissolved, leaving no color streaks, little or no haze, etc. in the fabricated product. They also preferably exhibit good lightfastness and heat stability and are resistant to migration or sublimation.
Fluorescent molecules absorb light at one wavelength and emit light at another, longer wavelength. When fluorescent molecules absorb a photon of a specific wavelength, an electron in a given orbital rises to a higher energy level (the excited) state. Electrons in this state are unstable and will return to the ground state, releasing energy in the form of light and heat. This emission of energy in the form of light is fluorescence. Because some energy is lost as heat, the emitted light contains less energy and therefore has a longer wavelength than the absorbed (or excitation) light. The notable shift between excitation stimulus and emitted light is called Stokes shift. The Stokes shift is typically considered long when it exceeds 50 nm.
Some fluorescent dyes are also useful for authentication as a covert additive. In such a usage, the fluorescent dyes are invisible under typical lighting conditions, yet brightly colored when illuminated with ultraviolet light.
3-hydroxychromone dyes range in color from green to orange. Some can be used in engineering thermoplastic polymers, such as polycarbonates, if their thermal stability is adequate. However, 3-hydroxychromones dyes are generally reported to possess poor lightfastness such that over a short period of time, the compound photodegrades resulting in a loss of fluorescence.
3-hydroxychromone dyes are known in the art; see e.g. U.S. Pat. Pub. 2006/0131761 entitled “COMPOSITION, METHOD OF AUTHENTICATING, METHODS OF MAKING AUTHENTICATABLE COMPOSITIONS, AUTHENTICATABLE ARTICLES MADE THERE FROM”, which is fully incorporated herein by reference. They are a subset of dyes having the general structure shown below:

wherein R1 is one of —O— or —NR6—;
R2 is selected from the group consisting of an aromatic radical having 3 to 30 carbon atoms, and a cycloaliphatic radical having 3 to 12 carbon atoms;
R3 is hydrogen or a labile group with the proviso that R3 is not a group selected from the group consisting of —CH2—(CH2)n—CH3 and —CH2—C6H5 wherein n has a value 0, 1, or 2;
R4 and R5 are either:                (i) independently selected from the group consisting of an aliphatic radical having 1 to 12 carbon atoms, an aromatic radical having 3 to 20 carbon atoms, a cycloaliphatic radical having 3 to 12 carbon atoms, a cyano group, a nitro group, a halo group, and a —OR7 group, wherein R7 is selected from the group consisting of hydrogen, an aliphatic radical having 1 to 12 carbon atoms, an aromatic radical having 3 to 20 carbon atoms, and a cycloaliphatic radical having 3 to 12 carbon atoms; or        (ii) together represent an aromatic radical having 3 to 12 carbon atoms, a heteroaromatic radical having 3 to 12 carbon atoms, or a pyranone radical of the formula        
                wherein R8 is selected from the same options as R1, R9 is selected from the same options as R2, and R10 is selected from the same options as R3; and        
R6 is selected from the group consisting of hydrogen, an aliphatic radical having 1 to 12 carbon atoms, an aromatic radical having 3 to 20 carbon atoms, and a cycloaliphatic radical having 3 to 12 carbon atoms.
When R1 is —O—, the structure is a 3-hydroxychromone. Hydroxy chromones typically have Stokes shifts greater than 100 nm, which are very long.
Some dyes in this class exhibit a color shift from a clear color to a visible color after exposure to UV radiation for a short period of time. This color shift occurs within several hours when exposed to both sunlight and xenon arc. The color shift is irreversible and the once covert additive is now visible to the viewer. Interestingly, this color shift only affects the color of the dye when viewed under typical lighting conditions; when stimulated with ultraviolet light, it still fluoresces, though with continued exposure fluorescence emission diminishes. While poor lightfastness is known for 3-hydroxychromone dyes, it is not generally associated with rapid photoyellowing (the creation of an absorbing chromophore).
One such dye which exhibits this color shift is BP-3-HF, a 3-hydroxychromone dye having the following formula:
BP-3-HF shifts from a clear color to a dark yellow-orange color after exposure to UV radiation for a short period of time. BP-3-HF is also known as 3-hydroxy-2-(4-biphenyl)-chromen-4-one.
Another such dye which exhibits this color shift is D-3-HF, a dye having the following formula:
D-3-HF shifts from a clear color to an amber color after exposure to UV radiation. D-3-HF is also known as 3,7-dihydroxy-2,8-diphenyl-4H, 6H-pyrano[3,2-g]chromene-4,6-dione.
Products are generally expected to retain their initial color when properly used. Because polycarbonates containing such dyes rapidly develop color upon UV absorption, and because the change in the color due to the color shift far exceeds the change in color due to resin photochemical degradation, commercial use of such dyes is highly constrained. Indeed, short-term exposure to sunlight during shipping/handling or to sunlight transmitted through window glass will likely provide sufficient energy to initiate photoyellowing.
It would be desirable to find a composition which allows for the use of such dyes, but provides good color retention and permits strong initial fluorescence emission intensity.